#include<iostream>
#include<math.h>
using namespace std;

double a, b, eps, h, t1, t0, s, n;

double f(double x){
    return 2/(1+x*x);
}

int main(){
    scanf("%lf %lf %lf", &a, &b, &eps);
    h = (b-a)/2;
    t1 = (f(a)+f(b))/2;
    n = 1;
    while(1){
        t0 = t1;
        s = 0;
        for(int k =1; k<= n; k++){
            s = s+ f(a+(2*k-1)*h/n);
        }
        t1 = t0/2 + s * h / n;
        if(abs(t1-t0)< 3 * eps){
            printf("t1:%.19lf\n", t1);
            break;
        }
        else{
            n * = 2.0;
        }
    }
}
